1. Field of the Invention
The present invention relates in general to power seat slide devices, and more particularly to power seat slide devices of a type which includes a threaded shaft that is rotatably supported by a movable rail, and which is driven by an electric motor and a nut that is fixed to a stationary rail and operatively engaged with the threaded shaft.
2. Description of the Prior Art
In order to clarify the task of the present invention, one conventional power seat slide device will be outlined with reference to FIG. 4 of the accompanying drawings.
As is seen from FIG. 4, a pair of parallel stationary rails 10 (only one is shown) are fixed to a vehicular floor (not shown). A pair of parallel movable rails 12 (only one is shown) mount thereon a seat (not shown) and slidably engage with the stationary rails 10. A front portion of an upper wall portion 13 of one movable rail 12 is somewhat raised for the purpose which will be described hereinafter. An elongate tunnel-like space 14 is defined by the engaged stationary and movable rails 10 and 12.
An electric motor (not shown) is mounted on the movable rail 12 and connected to a worm 16 through a drive shaft (not shown) for driving the worm 16. A worm wheel 18 has front and rear cylindrical portions 20 and 22 which extend forwardly and rearwardly therefrom, respectively. The worm wheel 18 has a rectangular through hole (not shown) formed therein which extends from a front end of the front cylindrical portion 20 to a rear end of the rear cylindrical portion 22. The worm 16 is operatively meshed with the worm wheel 18 so that rotation of the worm 16 drives the worm wheel 18. The worm 16 and the worm wheel 18 are housed in a gear housing 24 which is placed under the raised front portion of the upper wall portion 13 of the movable rail 12.
The gear housing 24 has an axially extending through hole 26 for receiving and rotatably supporting the worm wheel 18 and a laterally extending hole (not shown) for receiving and rotatably supporting the worm 16. The gear housing 24 is formed at its upper wall portion with two externally threaded cylindrical projections 30. The gear housing 24 is tightly fixed to the upper wall portion 13 of the movable rail 12 in such a manner that the threaded projections 30 are received in through holes 32 of the movable rail 12 and engaged with nuts 34.
A threaded shaft 28 which extends in and along the tunnel-like space 14, has a thinner front portion. The thinner portion has a rectangular portion 36 and a threaded front portion. The thinner portion of the threaded shaft 28 is received in the rectangular through hole of the worm wheel 18 in such a manner that the rectangular portion 36 of the threaded shaft 28 is fitted in the rectangular through hole of the worm wheel 18 to achieve integral rotation therebetween. A nut 38 is engaged with the threaded front portion of the threaded shaft 28. The threaded shaft 28 is rotatably supported at its rear end (not shown) by the movable rail 12 through a suitable holder (not shown) and operatively engaged with a nut 40 which is fixed to a bottom wall portion 42 of the stationary rail 10.
In the following paragraphs, the operation of the conventional power seat slide device will be described. The description will be commenced with respect to a condition wherein the seat, viz., the movable rail 12 fixed thereto assumes its rearmost position,
Under this condition, the electric motor is at its standstill position, and the seat is substantially locked at the rearmost position due to the nature of the meshed engagement between the fixed nut 40 and the threaded shaft 28. That is, under this condition, there is no relative movement between the threaded shaft 28, supported by the movable rail 12, and the nut 40 secured to the stationary rail 10. Even when, by for example a passenger seated on the seat, a certain force is suddenly applied to the threaded shaft 28 in a direction parallel with the axis of the threaded shaft 28, the meshed engagement between the nut 40 and the threaded shaft 28 prevents the threaded shaft 28 from making a movement relative to the fixed nut 40.
When now the motor is energized to rotate in a given direction, the power of the drive shaft of the motor is transmitted through the worm 16 and the worm wheel 18, to the threaded shaft 28, thereby rotating the same about its axis. With this rotation, the threaded shaft 28 advances forward relative to the fixed nut 40, and thus the movable rail 12 carrying the threaded shaft 28 is moved forward relative to the stationary rail 10. This means that the seat is moved forward.
When the seat comes to a desired position, the electric motor is deenergized. With this, the seat is is in a stationary place or is locked at the desired position.
However, the conventional power seat slide device as mentioned hereinabove has the following drawback.
In the case of a heavy head-on collision and the like, the seat together with the movable rail 12, is subject to a remarkable inertial force in a direction to move the seat and movable rail 12 forward relative to the stationary rail 10. Upon this occurrence, the threaded shaft 28, which is connected to the stationary rail 10 through the nut 40, tends to move rearward relative to the gear housing 24 which moves forward together with the movable rail 12. Therefore, mutually opposed movements are produced between the threaded shaft 28 and the gear housing 24. Thus, materials having high strength must be used for the gear case 24 for enduring the load applied thereto by the threaded shaft 28. Thus, the selection of the material to make up the gear case 24 is limited, and it is difficult to use a material such as plastic which is economical and easy to be molded for the gear housing 24.